1. Field of the Invention
The present invention relates to a technique for correcting the tone characteristic of an image.
2. Description of the Related Art
In recent years, digitization and resolution enhancement of videos have advanced. For example, it is demanded to display videos with higher quality even on televisions and displays for personal computers. For this reason, in a display, various kinds of image processing operations required to display an input video to have higher quality are executed.
As one technique for displaying a video to have higher quality, a technique called a dynamic gamma technique, which dynamically executes tone correction of a video, is known. In dynamic tone correction processing, processing for extracting an arbitrary feature amount from an input video, and generating a tone correction function based on the extracted feature amount is executed.
For example, a method of extracting a luminance histogram as a feature amount of an input video, and generating a function of converting a tone correction characteristic so as to average the values of the histogram in the luminance direction, or a table used to convert the tone correction characteristic is generally known.
However, if extreme correction is applied upon execution of dynamic tone correction, an unnatural video is often obtained. When a correction value is largely changed within a short period of time, the frame appears to flicker, or the brightness of a still part, such as a background in the frame, varies along with an elapse of time, resulting in an unnatural video. In order to solve these problems, various inventions are disclosed.
For example, in the invention disclosed in patent reference 1, the degree of scene changes is calculated based on a feature amount extracted from a video, and the response speed of tone correction is changed accordingly, thus improving display quality.    [Patent Reference 1] Japanese Patent Laid-Open No. 2006-270417
Another problem is posed upon execution of the dynamic tone correction processing. For example, assume that a tone correction curve is changed when only a part of a video moves or changes or in a scene in which the overall frame slowly moves by, e.g., a panning operation for moving a camera to capture an image. In this case, the brightness appears to change even for a non-moving part in a video or a panned video part, resulting in an unnatural video.
As one of visual effects, a scene called “fade-in” is known. For example, in a certain scene, the entire frame is displayed to have a black level in an initial scene, the luminance increases in steps along with an elapse of time, and an image gradually appears. In this case, since the conventional tone correction curve change method executes processing for increasing the luminance for a low-luminance image, it adversely affects an original fade-in effect.
The same applies to a fade-out effect. As an example of a fade-out scene, a normal image is displayed in an initial scene, the minimum level of the luminance of an image increases in steps along with an elapse of time, and the entire frame is finally displayed in white. In this case as well, since the conventional tone correction curve change method executes processing for lowering the luminance in case of an image with a high minimum luminance level, it adversely affects an original fade-out effect.